The present invention relates to wireless products, and more particularly relates to an RF system capable of adapting its bandwidth as needed by the system even where the signal bandwidth is less than a minimum bandwidth requirement of 47 CFR §15.247 for spread spectrum operation.
RF communications systems are known for transmitting data from an endpoint device to a receiver or intermediate transceiver. Home security systems and related software application programs use such RE systems to communicate messages in a form of RF signals. For example, in one known RF system application, an RF message comprising a signal generated by a tripped sensor located at a window in a secure location may be communicated by an RF transceiver. The signal (RF message indicating the alarm event) is relayed by the RE transceiver to a central control, which in turn relay the RF message to a security-monitoring center. In another application, RF transceivers are used in medical applications to communicate (upload or download) acquired medical data via telemetry from an implantable medical device.
RF transceivers and related communications systems are also known to provide wireless links to communications networks, for example, RF communications to access an Internet Service Provider (ISP). In one such application, wireless ISPs (WISPS) provide Internet service to those customers that do not have access to cable or to high speed DSL (digital subscriber line) Internet service. The need for such a WISP occurs, for example, where the major cable service provider in the area of those customers has not laid the “last mile” of cable (or the telephone operating company does not provide DSL service) to the customer.
In 1985, as an attempt to stimulate the production and use of wireless network products, the FCC modified Part 15 of the radio spectrum regulation (as per 47 C.F.R.), which governs unlicensed radiating devices, which include RF transceivers. The modification to part 15 authorized wireless network products to operate in the industrial, scientific, and medical (ISM) bands using spread spectrum modulation. The ISM frequencies that may be used include 902 to 928 MHz, 2.4 to 2.4835 GHz, and 5.725 to 5.850 GHz. The FCC thereby allows users to operate wireless products, such as utility metering systems, without obtaining FCC licenses if the products meet certain requirements. This additional flexibility in the use of the frequency spectrum eliminates the need for the user organizations to perform costly and time-consuming frequency planning that would be necessary for coordinating radio installations in such a way that they maintain the standard so that their operation does not interfere with existing radio system transmissions, for example, between 902 and 928 MHz.
Spread spectrum modulators use one of two methods to spread the signal over a wider area. The first method is that of direct sequence spread spectrum, or DSSS, while the second is frequency hopping spread spectrum, or FHSS. DSSS has potentially higher data transmission rates but is more costly than FHSS to implement. DSSS consumes more power and is more susceptible to noise that FHSS. FHSS is more advantageous that DSSS by its ability to avoid using selected (narrowband) channels within the overall allocated frequency band.
DSSS combines a data signal at the sending station with a higher data rate bit sequence, creating what is sometimes referred to as a chipping code (also known as a processing gain). A high processing gain increases the signals resistance to interference. Source data to be transmitted is first exclusive OR-ed with a pseudorandom variable sequence, to enlarge the sequence of the baseband data signal (sequence) to be greater than the source data rate. When the exclusive OR-ed signal is modulated and transmitted, it occupies, and is said to be spread over, a proportionally wider frequency band that the original source data bandwidth. The baseband signal spread in this way appears as pseudo noise to other users of the same frequency band.
FHSS operates by dividing an allocated frequency band into a number channels. Each channel is typically of equal bandwidth, which is determined by the data bit rate and the modulation method used. A transmitter then uses each channel for a short period of time before moving (hopping) to a different channel. When a channel is being used, RF carrier is modulated with the bits being transmitted at that time (in that channel). The channel pattern of usage is known as the hopping sequence. The time spent transmitting within each channel is known as the dwell time.
FHSS relies on the distribution of an RF data signal randomly hopped across a number of defined frequency channels to avoid interference. The hopping sequence determines when and in which order the channels are used for transmission. To properly receive the RF signal, the receiver must follow the same hoping sequence and listen to the incoming signal at the right time and on the correct channel (frequency). If the system encounters interference on one frequency, then the signal (comprising the channel's intended transmission) is retransmitted on a subsequent hop on another frequency. Because of the nature of its modulation technique, FHSS can achieve up to 2 Mbps data rates. The data throughput, however, decreases as interference increases because data is lost and must be retransmitted.
In 2002, the FCC revised Part 15 of the radio spectrum regulation (47 CFR), to provide for the introduction of new digital transmission technologies, thereby creating a new category, called digital modulation, which replaces and subsumes the more limited direct sequence spread spectrum (DSSS) category. 47 CFR §15.247.d reads “[f]or digitally modulated systems, the peak power spectral density conducted from the intentional radiator shall not be greater than 8 dBm in any 3 kHz band during any time interval of continuous transmission.” Paragraph 15.247.a.2 defines a digitally modulated system as one with a 6 dB bandwidth drop-off at least 500 kHz (wide).
The digital modulation systems are subject to the same power output maximum, which is 1 Watt and power spectral density limits of 8 dBm per 3 kHz, but without the same processing gain constraints as in the prior DSSS category.
These revisions provide an opportunity to improve the effectiveness of systems conforming to 47 CFR §15.247. For example, a wideband FSK transmitter (transceiver) with data spreading can now be used to transmit at power levels up to +12 dBm. This increased output power allows the wireless communication system and RF transceiver to perform better and with much lower cost and complexity than before the revisions. Both frequency hopping (FHSS) and direct sequence spread spectrum (DSSS), however, require the receiver to recover the transmitter's timing reference so that the receiver and transmitter are “in phase”. This added complication makes these techniques more expensive, less power efficient, and more complicated to design. Other such opportunities exist and are the basis for the present invention.